The problem of disposing of bulky waste materials is receiving increasing attention as existing landfills reach capacity and the availability of additional land for waste disposal decreases. Reducing waste, such as tires, in size permits volume densification and reduces the requirements of subsequent processing.
Shredder machines which utilize paired shearing wheels to shred waste material into smaller pieces have been developed. The term "shredder" as used herein means a machine which reduces objects by shearing action. For example, U.S. Pat. Nos. 5,145,120, 4,901,929 and 4,607,800 to Barclay disclose shredders in which rotating shearing wheels overlap at the edges of cutting segments, or shear knives, on the wheels to cut into the waste material like giant knives. Other patents teaching this type of machine include U.S. Pat. No. 4,374,573 to Rouse et al., U.S. Pat. No. 3,991,944 to Baikoff and U.S. Pat. No. 3,931,935 to Holman. The aforementioned machines are "primary" shredders in the sense that whole tires may be fed into the machines for shredding.
One problem faced by the aforementioned shredders is the relatively short operational life of the shear knives, due to the arduous nature of shearing waste material, e.g., tires. The wear caused by shearing reduces the edge of the shear knives, requiring periodic replacement of the blades. One approach to decreasing the periodicity of replacing the knives is to form them from exotic alloys. In this manner, the operational life of the knives are increased in that the knives hold a sharp edge for a greater period of time. This is typically unfeasible commercially, as the alloys employed are very expensive.
Another approach is to provide demountable shear knives that may be periodically resharpened. The above-cited patent to Rouse et al. teaches a shredder having paired shearing wheels with overlapping resharpenable shear knives along a periphery of each shearing wheel. However, the resharpening requires removal of material along the overlapping adjacent edges, so that the clearance between the edges is affected. This reduces the effectiveness of the shredder because the clearance between overlapping adjacent edges of the shear knives of the two shearing wheels must remain within a relatively small range. Consequently, the shear knives in Rouse et al. must be replaced regularly.
The above-cited U.S. Pat. No. 4,901,929 to Barclay overcomes the problem encountered by Rouse et al. by providing shims between an annular member and shear knives attached thereto. Each shear member has a radially outward end that is comprised of a center annular member extending from a hub and sandwiched between two resharpenable shear knives. To overcome the loss of material due to resharpening, a shim is placed between the center annular member and the resharpened shear knife. In this manner, the clearance between adjacent shear knives is maintained. However, Barclay '929 has demonstrated undue clearance in the knife due to degeneration of the shim or annular member.
It is an object, therefore, of the present invention to provide a shear knife for a rotary shearing wheel that has an longer operational life than the shear knives of the prior art.